This invention relates to a hearing device comprising at least a signal processing unit and at least a fuel cell to supply the signal processing unit with energy. Hearing devices may be e.g. behind-the-ear hearing aids or in-the-ear hearing aids or hearing instruments.
Hearing aids or hearing instruments are usually powered with batteries, namely zinc-air batteries, due to their relatively high energy-to-volume ratio. The availability and cost of these batteries are significant factors in the cost-of-ownership of hearing aids or hearing instruments however. That is the reason why accumulators are not used in large numbers due to their comparative low energy storage capacity. Although improvements in storage capacity in batteries as well as in accumulators are ongoing, no significant increases in view of factors or orders of magnitude can be expected.
Other means such as solar power (U.S. Pat. No. 5,303,305) or the use of body heat (DE 19 530 382) has been investigated, but such methods generate too little energy for a reasonable usage in hearing aids or hearing instruments.
The use of fuel cells for hearing instruments is described in DE 10 115 429 with low temperature fuel cells based on methanol as fuel. The benefit of this type of energy storage is based on its significantly higher capacity which allows either less refilling or more applications requiring higher power consumption.
Nevertheless, the fuel cell concept of DE 10 115 429 suffers from several disadvantages. If the fuel tank is arranged detachable from the fuel cell within the hearing device, mechanical connections are required, which are prone to break and space is needed for the connection elements, and furthermore electrical contacts are required, which are prone to oxidize. If the fuel cell is integrated in the hearing device, a spacious mechanical vent for refilling is required, as known from cigarette lighters, with the further disadvantage of being difficult to handle, especially for elderly people.
Furthermore, such a fuel tank needs to be made of at least partly transparent material in order to optically check its filling state. But even with transparent material, such an optical check needs a certain light condition with a certain minimum amount of ambient light for the recognition of the filling state. The transparent material must further be biocompatible for the use in hearing devices, especially for in-the-ear hearing devices, and has to be worked together with the other shell material.
To accurately estimate the filling state of the fuel cell, a regular shape of the tank is required, thereby limiting the degree of freedom for the integration of the tank within the small volume of a hearing device shell. A further problem is to be seen by surface tension effects on such small tank volumes, of miniaturized devices such as small hearing devices, which may mislead the user about the actual filling state.